Individuals are selected, but populations evolve Population genetics Modern synthesis Population...

Individuals are selected, but populations evolve

• Population genetics

• Modern synthesis

• Population• Species• Gene pool

Fir populations Not totally isolated, interbreeding may occur

Eastern U.S. People mobile, but more likely to choose mate locally

• Gene pool – all alleles in a population• Diploid organisms – 2 alleles at each locus• Two or more alleles in a population – each

has a relative frequency (proportion)• Population = 500 organisms = ? alleles• 320 homozygous dominant = ? alleles• 160 heterozygous = ? alleles• 20 homozygous recessive = ? alleles

HARDY - WEINBERGp + q = 1p2 + 2pq + q2 = 1 p = frequency of dominant alleleq = frequency of recessive allele

PKU 1/10,000 births; q2 = ?

Hardy-Weinberg Equilibrium

Very large populationNo migrationNo net mutationsRandom matingNo natural selectionNo genetic drift

Causes of MicroevolutionEvolution is a generation-to-generation

change in a population’s frequencies of alleles.

Genetic Drift – a change in a population’s allele frequencies due to chance bottleneck effect: drastic reduction in population size founder effect: new colony, not representative of the

larger populationNatural Selection – allele frequencies in offspring

generation different than parental due to differential reproductive success

GENE FLOW: genetic change due to migration, tends to reduce differences between populations.

Gene flow & Human Evolution

MUTATION: a change in an organism’s DNA

Quantitative characters - vary along continuum

Discrete characters - either/or

Polymorphism - 2+ forms represented - applies to discrete characters

Gene diversity - average % heterozygous loci

Nucleotide diversity - about 0.01% in humans

Clinal variation

GEOGRAPHIC VARIATION

Madeira: 2 mouse populations separated by mountains

Squirrel populations & the Grand Canyon

• DIPLOIDY – recessive alleles cannot be selected for/against in heterozygotes

• BALANCED POLYMORPHISM– Heterozygote advantage: malaria &

sickle cell anemia– Frequency dependent selection:

survival & reproduction of 1 morph declines if that phenotype becomes too common

• NEUTRAL VARIATION

FREQUENCY DEPENDENT SELECTION IN HOST/PARASITE RELATIONSHIP

Modes of selection

Directional selection

Intermediate selected against – inefficient at cracking both size seeds

Sex: 2 fold disadvantage

Sexual SelectionSexual dimorphism: secondary sex

characteristicsIntrasexual selection: direct competition

among same sex individuals Intersexual selection: mate choice,

individuals of one sex (usually female) are choosy in selecting mates